1. Field of the Invention
The invention relates to phase change switches, and more particularly, to phase change switches having a dynamic range of impedance. More specifically, the invention relates to such switches which can be employed in circuits such as on frequency selective surface arrays, for controlling current flow throughout the array, through the use of the switches. By controlling such current flow, the properties of the frequency selective surface array can be actively controlled.
2. Background of the Invention
A two-dimensional periodic array of patch or aperture elements is called a frequency selective surface (FSS) because of the frequency selective transmission and reflection properties of the structure. In the past, many FSS applications and sophisticated analytical techniques have emerged. Applications include multi-band FSS, reflector antennas, phased array antennas, and bandpass radomes.
More recently, capabilities of the FSS have been extended by the addition of active devices embedded into the unit cell of the periodic structure. Such structures are generally known as active grid arrays.
Active grid arrays have been developed in which a variable impedance element is incorporated to provide an FSS whose characteristics are externally controllable. However, such applications involve complex structures that can be difficult to manufacture and control.
Mechanical on/off switches have been used in circuits designed to interact with electromagnetic waves. The mechanical process in these on/off switches involves the physical motion of a conductor between two positions, i.e., one where the bridge touches another conductor and completes the conducting path of the circuit, and the other where it has moved away from the contact to break the circuit paths. Such mechanical switches have been made at micrometer size scale. The capacitances between the two switch conductors in the open or “off” position must be lowered to a level that effectively breaks the circuit for alternating electromagnetic current flow.
Alternatively, transistor and transistor-like semiconductor switching devices have been used in circuits designed to interact with electromagnetic waves. However, for the specific applications herein, conventional semiconductor switching devices typically will not operate to open and close circuits effectively to electromagnetic current flow in the frequency range of terahertz and above because at these frequencies, various intrinsic capacitances in the device structure can provide low impedance circuit paths that prevent the switch from operating as intended.
In the field of semiconductor memory devices, it has been proposed to use a reversible structural phase change (from amorphous to crystalline phase) thin-film chalcogenide alloy material as a data storage mechanism. A small volume of alloy in each memory cell acts as a fast programmable resistor, switching between high and low resistance states. The phase state of the alloy material is switched by application of a current pulse. The cell is bi-stable, i.e., it remains (with no application of signal or energy required) in the last state into which it was switched until the next current pulse of sufficient magnitude is applied.